>perl -wMstrict -le
"my $test = 'abc1 abc1 abc1 abc1 abc1';
 $test =~ s/1/++$1/eg;
 print $test;
"
Modification of a read-only value attempted at -e line 1.

##</code><code>##

>perl -wMstrict -le
"my $addend = 0;
 my $test = 'abc1 abc1 abc1 abc1 abc1';
 $test =~ s/(1)/$addend++ + $1/eg;
 print $test;
"
abc1 abc2 abc3 abc4 abc5